This application claims priority to German application No. DE 102 08 117.4, filed Feb. 26, 2002, which application is incorporated herein by specific reference.
1. The Field of the Invention
The present invention relates to a transimpedance amplifier of the kind used, for example, in an integrated preamplifier circuit of an optical receiver module in order to amplify and convert into voltage the data signals transmitted via an optical waveguide and converted into corresponding current pulses by a photodiode.
2. The Relevant Technology
FIG. 3 shows by way of example the structure of a conventional transimpedance amplifier. The transimpedance amplifier includes an input stage 1 and an output stage 2 coupled thereto. The input stage 1 includes as the amplifying element a bipolar transistor Q1 the collector of which is connected to a supply voltage Vcc via a resistor Rc. The output stage 2 likewise includes an amplifying element Q2 in the form of a bipolar transistor the collector of which is connected to the supply voltage Vcc. The base of the transistor Q2 is connected to the collector of the transistor Q1. An input signal in the form of an input current is fed to the base of transistor Q1 while an amplified output voltage uo can be tapped at the emitter of transistor Q2 via an emitter resistor Rc. The voltage applied to the base of transistor Q1 is designated ui, reference being made below to the voltage ui with regard to the calculation of operating points and for quantification of the voltage/voltage amplification of the non-back-coupled amplifier (Open Loop Amplification). The base of transistor Q1 of the input stage 1 therefore forms an input connection of the transimpedance amplifier, while the emitter of transistor Q2 of the output stage 2 represents an output connection of the transimpedance amplifier, said output connection being back-coupled via a feedback loop 3 including a transimpedance resistor Rt to the input connection or the base of transistor Q1. A feedback current corresponding to the output voltage uo is therefore supplied to the base of transistor Q1 via said transimpedance resistor Rt.
The feedback effected by the transimpedance resistor Rt serves to set an operating point of the transimpedance amplifier which is as stable as possible, the current/voltage amplification of the transimpedance amplifier being determined by the value of the transimpedance resistor Rt. The open loop amplification Aol of the transimpedance amplifier shown in FIG. 3 is calculated as follows:                                                                         A                ol                            =                              xe2x80x83                            ⁢                                                u                  o                                                  u                  t                                                                                                        =                              xe2x80x83                            ⁢                                                g                  m                                ·                                  R                  c                                                                                                        =                              xe2x80x83                            ⁢                                                                    I                    c                                                        U                    t                                                  ·                                  R                  c                                                                                                        =                              xe2x80x83                            ⁢                                                                                                                  V                        cc                                            -                                              2                        ·                                                  U                          be                                                                                                            R                      c                                                        ·                                      1                                          U                      t                                                        ·                                      R                    c                                                  =                                                                                                    V                        cc                                            -                                              2                        ·                                                  U                          bc                                                                                                            U                      t                                                        ,                                                                                        (        1        )            
where gm denotes the transconductance of the amplifier, Ube the base-emitter voltage of the transistors Q1, Q2 and Ut the so-called temperature voltage, which is dependent on the ambient temperature. It can be seen from the above formula (1) that the open loop amplification of the transimpedance amplifier and therefore its frequency behaviour depend on the ambient temperature and the supply voltage. This dependence of the open loop amplification on ambient temperature and supply voltage can lead to instabilities in the amplifier circuit when there is feedback, since the supply voltage and ambient temperature can fluctuate, depending on the application concerned.
A possible solution to the above-mentioned problem consists in dimensioning the transimpedance amplifier in such a way that the desired bandwidth can be achieved at all possible temperature and supply voltage values. To achieve this with regard to bandwidth, however, overdimensioning of the transimpedance amplifier is required, which is disadvantageous with regard both to the noise properties and to the stability of the transimpedance amplifier.
Known from xe2x80x9cIntegrated Circuits for a 200-Mbit/s Fiber-Optic Linkxe2x80x9d, Michael P. Cook, Geoff W. Sumerling, Tran van Muoi, Andy C. Carter, IEEE Journal of Solid State Circuits, vol. SC-21, No. 6, December 1986, pages 909-915 is a transimpedance amplifier for a preamplifier circuit in a receiver module of an optical data transmission network, said transimpedance amplifier having a cascode circuit between the input and output stages. The cascode current flowing via this cascode circuit is indirectly monitored via an identically structured dummy cascode circuit and converted into a corresponding voltage by means of a resistor connected in series thereto, which voltage is in turn compared to a reference voltage dependent on ambient temperature. The bias voltage of the transimpedance amplifier, which thus influences the cascode current, is controlled as a function of the result of the comparison. In this way the cascode current can be adjusted proportionally to the absolute temperature, so that the dynamic impedance of the base-emitter diode of the bipolar transistor of the input stage can be kept constant and an amplification of the transimpedance amplifier independent of ambient temperature can be achieved.
However, through the use of the cascode circuit the control concept of the above-described transimpedance amplifier is associated with a relatively high implementation cost, because, among other reasons, additional circuit sections for applying the bias voltage to the cascode circuit are required. In addition, in this known transimpedance amplifier, the operating point setting (biasing) of the dummy cascode circuit is effected by using voltages of the active transimpedance stage, which is detrimental to the frequency behaviour of this stage.
It is therefore the object of the present invention to propose a transimpedance amplifier whereby the above-described disadvantages are eliminated and whereby (open loop) amplification of the transimpedance amplifier independent of temperature and supply voltage fluctuations can be achieved at low implementation cost.
It is proposed according to the invention, by means of a suitable current control circuit, to detect the current flowing via the amplifying element or the bipolar transistor of the input stage and to control said current in such a way that it is independent of the ambient temperature. At the same time, it can thereby be achieved that the current controlled is independent of fluctuations of the supply voltage.
To achieve the above-mentioned effect the current flowing in the input stage can be used suitably to control the control or base voltage of a bipolar transistor coupled to the amplifying element of the input stage.
Because direct measurement of the current flowing in the input stage would be detrimental to the frequency behaviour of the transimpedance amplifier a dummy amplifier (replica amplifier) which emulates the behaviour of the whole transimpedance amplifier is preferably used, in which case, instead of the current flowing in the input stage of the transimpedance amplifier itself, the current flowing in the input stage of the dummy amplifier connected in parallel thereto is detected and used to control the current flowing in the input stage of the actual transimpedance amplifier. Detection of the current flowing in the input stage of the dummy amplifier is effected preferably by means of a current mirror circuit.
The transimpedance amplifier according to the invention can be used, for example, in a preamplifier circuit of an optical receiver module, whereby the data or information signals received by a photodiode via an optical waveguide in case of large bandwidths can, by means of a preamplifier circuit of this kind, be amplified and converted into voltage for further processing.
Because, in the case of the transimpedance amplifier according to the invention, an operating point setting once selected is valid for all possible temperature and supply voltage values, a significant reduction in design time can be achieved. To take account of extreme operating conditions, at most a small overdimensioning with regard to bandwidth is required. Because of the reduced overdimensioning of bandwidth an improvement in the sensitivity and stability of the transimpedance amplifier circuit can be achieved.
Unlike the case with the above-mentioned conventional transimpedance amplifier circuit, no cascode circuits are required, so that the circuit structure can be simplified and circuit complexity reduced. The biasing circuit stages otherwise required for a cascode circuit can be omitted. Because a replica of the whole amplifier stage is used as a dummy for current measurement, no internal nodal points are required in the active amplifier stage for current measurement, so that greater bandwidth and higher operating speed can be achieved. In addition, in comparison to the conventional transimpedance amplifier, which necessitates a cascode circuit, operation with smaller supply voltages is possible.
Because of the above-mentioned advantages of the transimpedance amplifier according to the invention, greater security in the design of the circuit can be achieved.